The invention relates to a driving belt, comprising at least one endless, flat, metal, carrying band for a number of transverse members which can move with respect to this band and of which the surface which is in contact with the carrying band shows a crosswise curvature.
Such a driving belt is known from Dutch patent application No. 75 11879.
It is well known that the carrier band of such a driving belt must be centered with respect to the band supporting surfaces provided in the transverse members. This because in transverse members provided with a single central supporting surface contact between the band with the side edges of the surface contact between the band and said edges and in a driving belt with two carrying bands, which lie at both sides of the axis of symmetry--an embodiment which has great advantages when assembling the transverse members on the carrying bands--because movement of said bands beyond the outer edges of the transverse members has a result that, when the transverse members move in between the pulleys, the band comes into contact with the pulley and is pulled out of the transverse members.
When the carrierband consists, as is usual, if a packet of thin bands, the other bands are drawn out of the transverse members too and within a very short time first the side of the driving belt and thereafter the whole driving belt is destroyed.
The abovementioned patent application proposes to obtain said centering by using supporting surface for the bands with a shape which is convex in the direction which is perpendicular to the carrying band. This is based upon the reasoning that then an effect is obtained which corresponds with the centering effect of a driving belt used in combination with and resting upon a pulley with convex curvature; the driving belt tends to move to the highest point until it remains centered there. Practice, however, has shown that the measure proposed in the above application does not always result into the desired effect: also when a convex supporting surface is used, a good centering of the carrying band or the packet of thin bands is not always ensured with the abovementioned results.
This can be explained as follows:
When the coefficient of friction between a band and the supporting surface is high, for instance in the absence of any lubrication or when the combination of the materials which are into contact with each other results into a high coefficient of friction, the centering effect is practically always there, also when the tension in the band is low. Vibrations do not influence this effect. Then the coefficient of friction is only medium and the surface is dry, then the centering effect will be there when there is a high tension in the band but vibrations will start to have a detrimental influence. When there are no tensions in the band the centering effect is only small.
When, however, the coefficient of friction between the band and the supporting surface is low, for instance when there is a good lubrication which will be the case when band and pulleys run in an oil bath or are sprayed with oil--which is necessary in a continuously variable transmission to obtain a high efficiency, and a certain measure of cooling--then with high tensions in the band the centering effect will not be reliable enough to ensure a centering under all circumstances. In practice there will always be operating conditions in which there is only little tension in the band (note that, when the transverse members are assembled on the band in such a way that there is a high initial tension this tension will drop after a number of operating hours as a result of plastic deformation and wear of the transverse members) and then the centering effect will not be present at all.